In an OFDMA/TDD scheme, a fast data signal is converted into a slow data signal and data is transmitted/received through a subchannel having a plurality of subcarriers.
Generally, a downlink refers to a scheme in which a signal is transmitted from one base station to a plurality of users. In this case, data signals from the users are allocated to a plurality of subchannels and then transmitted to a downlink.
At this time, if the individual user terminals receive the data signals transmitted from one base station, all of the subcarriers in one OFDM symbol simultaneously reach the terminal receivers with no time delay, thereby maintaining orthogonality among the subcarriers.
However, in an uplink, due to a temporal difference and a positional difference between the terminals, absolute time synchronization is not achieved among the data signals from the terminals that reach the base station.
If the absolute time synchronization is not achieved among the terminals, in one OFDM symbol having the subchannels from a plurality of terminals, signal orthogonality among the subcarriers constituting each subchannel may be lost, resulting in a loss of an original signal upon data demodulation in the base station.
To overcome this problem, the base station measures time delays from a base station reference time in respect to the individual signals transmitted from the terminals and adjusts terminal transmission time on the basis of the base station reference time. Then, the time delays due to the temporal difference and the positional difference between the terminals are equalized.
For the measurement of an uplink time delay between an individual terminal and the base station, the corresponding terminal generates a pseudo-random ranging code, then performs BPSK (Binary Phase Shift Keying) modulation, and subsequently transmits the modulated ranging code to the base station through an allocated ranging channel.
At this time, the ranging codes transmitted from a plurality of terminals through the ranging channels are transmitted to the base station with separate random code characteristics.
Further, the individual signals received by the base station through a radio channel have a separate time delay due to the positional difference between the terminals and a difference in wireless channel environment. In the base station, a signal corresponding to the time delay is represented in a complex exponential form, and the phase thereof is modulated with respect to the original signal.
Accordingly, the base station first sequentially multiplies all possible complex exponential values, which removes the timing error represented in the complex exponential form, to find a timing error with respect to the base station reference time, and removes the timing error. Next, the base station performs a process of calculating the correlation values of all possible pseudo-random ranging codes and comparing the calculated correlation values with a threshold value, thereby finding the timing error of each terminal with respect to the base station reference time and a corresponding ranging code.
When the complex exponential term is sequentially multiplied to find a complex exponential value corresponding to a correct timing error, the number of complex exponential searches may become excessively large. As a result, the system requirements for fast ranging response are not met.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.